Standardization efforts for the wireless LAN standard in IEEE 802.11 so far have mainly targeted indoor communication and has successively added physical layer standards mainly focusing on an increase in transmission capacity such as 802.11b (maximum 11 Mbps), 802.11a, 11g (maximum 54 Mbps), 802.11n (maximum 600 Mbps), and 802.11ac (maximum 6.9 Gbps). Meanwhile, as examination of smart meters to implement a smart grid has been well under way, the need for low rate and long-distance outdoor transmission is also increasing. There are also ongoing discussions about assignment of available specified low power radio frequencies and the like intended for such applications. Against this background, studies aiming at development of new communication standards using a sub GHz band (frequency band slightly lower than 1 GHz) have started and IEEE802.11 started up TGah (802.11ah), a task group working on wireless LAN standards using sub GHz frequency bands in 2010. A principal required specification in TGah (802.11ah) is “data rate of 100 kbps or higher and maximum transmission distance of 1 km.”
IEEE 802.11a and subsequent standards including TGah (802.11ah) using an OFDM modulation scheme establish various kinds of synchronization using a preamble at the beginning of a packet to perform burst communication. A preamble is constructed of an STF (Short Training Field, which may also be called “short preamble”) used for AGC (Automatic Gain Control) or coarse adjustment AFC (Automatic Frequency Control) and LTF (Long Training Field, which may also be called “long preamble”) used for estimation of fine adjustment AFC or transmission path characteristic.
In order to efficiently use limited frequency resources, such an operation may be adopted that a plurality of adjacent channels are generally used by a plurality of users. However, in such an operation, in a certain receiving apparatus, received power of adjacent channels may be greater than received power of the channel intended for the certain apparatus (subject channel) due to the influence of fading or the like. For this reason, the operation in which adjacent channels are used by a plurality of users defines a spectral mask of a transmission signal, provides a frequency margin (which may also be referred to as “guard band”) between channels and thereby reduces interference to the adjacent channels.
However, in an environment in which interference to the adjacent channels is large and there is such a DU ratio (desired signal to undesired signal ratio) that exceeds interference allowable by the spectral mask, guard band or the like, leakage power (which may also be referred to as “interference power or disturbance power”) from the adjacent channels is mixed with the subject channel and interference from the adjacent channels may be produced as frequency-selective interference.
FIG. 13 illustrates an example of adjacent channel interference. In FIG. 13, the horizontal axis represents a frequency (f) and the vertical axis represents power (P). As shown in FIG. 13, part of a signal spectrum of a frequency band of an adjacent channel interferes with a signal spectrum of a frequency band (transmission band) of the subject channel, producing frequency-selective interference. Reception characteristics deteriorate due to data assigned to subcarriers in frequency domain affected by such frequency-selective interference.
Especially, 802.11ah assumes a long-distance transmission environment as described above. For this reason, in a situation in which a transmitting apparatus that transmits a signal intended for the channel used for the apparatus (hereinafter, may be referred to as “subject channel”) is located far and a transmitting apparatus that transmits a signal intended for an adjacent channel is located near, the received power of the adjacent channel may be more likely to be greater than the received power of the subject channel.
As a method for reducing the influence of such frequency-selective interference, Patent Literature (hereinafter, referred to as “PTL”) 1 proposes a method in which a receiving apparatus detects a subcarrier affected by frequency-selective interference, using a pilot signal frequency-multiplexed with data, applies erasure processing to the data assigned to the subcarriers and applies error correction to the data.
In addition, PTL 2 and PTL 3 propose a method in which a receiving apparatus calculates the magnitude of a variance of a demodulated signal for each subcarrier to which data is assigned (hereinafter referred to as “data subcarrier”), detects a subcarrier affected by frequency-selective interference based on the calculated magnitude of variance, determines the degree of reliability of the demodulated signal of the subcarrier based on the magnitude of variance, makes a soft decision by gradually assigning weights to the demodulated signal based on the determined degree of reliability and performs error correction regarding the soft decision result.
PTL 4 proposes a method in which a receiving apparatus receives an OFDM signal made up of a plurality of subcarriers in which pilot signals are inserted according to a certain rule, extracts the pilot signals, calculates transmission path characteristics of the pilot signals, calculates an error signal as a time variation from the transmission path characteristics of the pilot signals and transmission path characteristics of pilot signals calculated one period earlier, detects the degree of reliability of subcarriers affected by frequency-selective interference based on the value of the error signal, makes a soft decision by assigning weights to the demodulated signal based on the degree of reliability and performs error correction regarding the soft decision result.
PTL 5 proposes a method in which a receiving apparatus receives an OFDM signal in which a null subcarrier (subcarrier to which no signal is assigned) is inserted together with data at a predetermined subcarrier position, calculates received power of the null subcarrier and determines whether or not interference has occurred at each subcarrier position according to the calculated received power of the null subcarriers.